1. Field of the Invention
The invention relates to a heat-sealable plastic film with at least a two-stage structure and with a thickness ranging up to 400 .mu.m, suitable for sealing plastic containers made, for instance, of polystyrene. The plastic film is particularly suitable for sealing containers that hold food items such as yoghurt cups.
2. Discussion of the Background
Products made of plastics are frequently used today instead of containers made of wood or such inorganic materials as metals, glass or ceramics. Requirements are always particularly high if the containers or vessels are used to hold and preserve food. A significant aspect of food preservation-whether accomplished by dehydration, freezing or sterilization, is the absolute prevention of bacterial growth. This requirement frequently calls for an airtight seal of the container. Mechanical stability, control of the moisture content, and the effects of atmosphere and of light must, moreover, also be considered for plastic containers. (See Ullmann, Encyclopedia of Industrial Chemistry, 25th Edition, Vol. A11, pp 523-560 and 583-618, VCH, 1988, where pertinent standards are also discussed).
Aluminum covers coated with a sealing varnish are often used to seal plastic containers, particularly if used for such dairy products as yoghurt cups.
Such aluminum covers are frequently made of a three-layered laminate, the external layer of which often consists of biaxially oriented polyethylene terapthalate (0-PET), biaxially oriented polypropylene (OPP) or polyamide (0-PA), or cellulose. The heat-sealable inner layer, on the other hand, usually consists of polyethylene, ethylene copolymers or of polypropylene (U.S. Pat. No. 4,753,708; G. Stehle, Neue Verpackung, September 91, pp 94-101) However, economic and ecological objections are increasingly being raised to the use of aluminum in food packages especially because of the high energy requirement for its production.
Plastic materials with a sealing coat are, therefore, also used instead of aluminum. The sealing coat is then adapted to the plastic material used. Polyvinyl chloride (hereinafter PVC) plays an important role here because it is a relatively cheap material for a sealing film that has good mechanical rigidity and acts as an effective barrier. The sealing coat usually consists of an acrylic resin, the adhesive character and melting point of which can be appropriately modified with additives.
However, there are some problems with the use of PVC (see Ullmann Encyclopedia, loc. cit., p. 593). Consequently, there a tendency to substitute other thermoplastic materials for disposable packages.
DE-A 3,531,036 describes plastic films that can be produced by coextrusion and consist of a sealable layer of impact-resistant polystyrene, a block copolymer and lubricant.
EP-A 0406,681, however, correctly points out the problems associated with substituting heat-sealable plastic film for aluminum foil. The significantly narrower processing range is usually a noticeable constraint. A very narrow processing range of 10.degree.-20.degree. C. must generally be maintained rather constantly to assure trouble-free production and the unproblematic use of the sealed container. In racking systems with several cavities for the simultaneous filling of cups, this requirement is not always met.
EP-A 0 406,681 addresses the problem of improving the procedure for producing films with a polystyrene or other base in accordance with the coextrusion system of DE-A 3,531,036 in such a way that processing range and processing reliability can be increased.
Furthermore, trouble-free production was to be assured also for filling systems with several filling cavities. In practice, this means using higher sealing temperatures with corresponding requirements concerning the quality of the plastic films.
According to EP-A 0 406,681 these requirements are satisfied by a sealable plastic film produced by a coextrusion or laminate process. Said film is composed of two layers, A and C and, if applicable, a layer B as well and optionally one layer each of a adhesion promoter D for joining each of the layers A, optionally B and C, consisting of a 1 to 50% of a layer of a heat-sealable and impact-resistant polystyrene A; up to 95% of a supporting base B; and 1 to 99% of a high melting plastic layer C, wherein the sum of the thickness or of the weight of A and, optionally B and C is always 100%.
While polystyrene as a packaging agent must be viewed favorably from the point of view of production costs and transformation requirements (110.degree. to 160.degree. C.), the relatively high permeability for gases and vapors can lead to problems. Although the problems created by adherence to the sealing heads can be countered by a layered construction of the sealing film with an impact-resistant polystyrene base, the solution creates increased manufacturing expenses and, possibly, waste-removal problems.
Thus, there remains the need for a suitable material for heat-sealable film. In particular, such a film should be suitable for creating an airtight seal for containers with a polystyrene base, if possible, with a homogenous layer and without additional treatment of the surface.
Above all, the film material should be environment-friendly, which means that one should be able to recycle it, even if it has a multi-layered structure.
It has now been discovered that the heat-sealable plastic films PF of the invention, although belonging to a different class of polymers, largely satisfy the technical requirements with regard to mechanical stability, chemical attributes, barrier characteristics, thermal characteristics, ease of processing (see Ullmann, Encyclopedia Vol. All, loc. cit.) and ease of recycling.